Bioactive materials are defined as materials which form a chemical bond with living tissue. Ceramic implant materials, such as alumina and calcium phosphate-based ceramics, may be bio-inert, biodegradable, resorbable, or bioactive depending upon their composition. True bonding between bone and man-made materials has been observed only within a limited group of ceramics, such as, the BIOGLASS.RTM. family, CERA-VITAL.TM., and apatite-wollastonite, all of which are based on ceramic oxide formulations.
Bioactive ceramic materials are believed to bond to bone because an oxide, or a ratio of oxides, present in the ceramic material permits a time-dependent, kinetic modification of the surface of the ceramic through an ion-exchange reaction that is triggered upon implantation into body fluids near bone or soft tissue. Upon implantation, a biologically-active calcium hydrocarbonate apatite (HA) layer forms at the surface of the ceramic. This HA layer is chemically and crystallographically equivalent to the mineral phase in bone. The equivalence between the ceramic HA layer and bone is considered to be responsible for the interfacial bonding between the ceramic implant and the bone or soft tissue. Specific to the bioactive glass-ceramic materials, the rate of release of calcium from apatite-wollastonite normally is constrained by the need to balance out-diffusion of cations and either phosphate or silicate ions in order to maintain charge neutrality.
Two problems currently exist which prevent widespread use of bioactive materials for implantation into soft or hard tissue, and for load bearing applications. These problems are (1) the extended time period required for significant hard or soft tissue-ceramic bonding following implantation, and (2) the ultimate bond strength achieved. These problems are present regardless of whether the material used for the implant is an amorphous bioactive material, such as the BIOGLASS.RTM. family, or a crystalline or semi-crystalline material, such as CERA-VITAL.TM. or apatite-wollastonite. However, the problems are especially critical when the material is a bioactive glass-ceramic material having a primarily crystalline structure. The glass-ceramics are stronger and better suited for load-bearing applications, but are significantly less bioactive than their glassy counterparts.
The kinetics of the ion-exchange reaction--that is, the release of critical ions (Ca, Si, Mg, Na, P, etc.) required to form a biologically-active HA layer--are believed to relate directly to both the rate of hard or soft tissue-ceramic bonding and to the strength of the resulting bond. Because phosphate ions present in body fluids can react with calcium to form HA, it is believed that the critical requirement for bonding and osseointegration of ceramics is the release of calcium.
In general, crystalline materials have stronger atomic bonds and increased stability as opposed to amorphous materials. As a result, the rate of the ion-exchange reaction for crystalline or semi-crystalline materials is significantly slower than for more amorphous materials under the same conditions. This significantly slower ion exchange reaction causes glass-ceramics to have a lower bioactivity than amorphous materials.
The rate of bonding between tissue and both crystalline and amorphous ceramics could be increased if a method could be found to increase the kinetics of this ion-exchange reaction. This would allow BIOGLASS.RTM. and the stronger bioactive glass-ceramics to become more bioactive, rendering the bioactive glass-ceramics suitable for appropriate load-bearing applications. An increase in the rate of these critical ion-exchange reactions also should result in a decreased period of patient convalescence, leading to a greater implant success rate, and an increased lifetime for ceramic implants.